The present invention relates to an improvement of a compact fluorescent lamp similar to an incandescent lamp in shape, or more in particular to an improvement of the discharge ignition characteristic of a fluorescent lamp comprising an outer bulb and a plurality of bent inner tubes forming a discharge path, each of the inner tubes having an opening at an end and an electrode provided at the other end thereof.
As an example of a compact fluorescent lamp of a shape similar to an incandescent lamp, an ordinary fluorescent lamp thinned and bent and housed together with an operating circuit or starter in an outer bulb of a shape similar to an incandescent lamp is in practical use.
In such a lamp, the temperature of the discharge tube generally is high, resulting in the vapor pressure of mercury being increased naturally. In order to maintain the vapor pressure of mercury at optimum condition, means such as providing amalgam in the tube have been taken in the bulb. Nevertheless, it is still difficult to produce a lamp of high output. An example of fluorescent lamps which are to some extent free from this problem is disclosed in U.S. Pat. No. 4,199,708. This fluorescent lamp comprises an outer bulb sealed in a gas tight manner, at least a pair of bent inner tubes, each of the inner tubes having an end opened into the outer bulb and the other end with an electrode, and a discharge path leading from the electrode of one inner tube through the opening thereof into the outer bulb, through the opening of the other inner tube to the electrode of the other inner tube. Although the temperature of the inner tubes of this fluorescent lamp is also high, the vapor pressure of mercury has an almost optimum value because the vapor pressure of mercury is dependent on the temperature of the coldest point of the outer bulb.
In this compact fluorescent lamp which comprises a plurality of inner tubes bent and housed in a comparatively small outer bulb, however, the inner tubes forming a discharge path are generally thin, resulting in an extremely high ignition voltage or starting voltage of the fluorescent lamp.